FIG. 1 depicts schematically parts of a power train for a motor vehicle 1, such as a passenger car or a heavy vehicle, e.g. a truck or bus. The power train comprises an engine 10 mechanically connected by a shaft to a first end of a gearbox 20 via a clutch device 40. The gearbox 20 is also mechanically connected, at its other end, by a propeller shaft 50 to a differential gear 30 associated with a rear axle. The rear axle comprises respective left and right drive shafts 60 which drive the vehicle's powered wheels (not depicted in the diagram).
With this well-known arrangement, the mechanical work of the engine 10 is transmitted via various transmission devices (e.g. clutch device 40, gearbox 20, propeller shaft 50, differential gear 30 and drive shafts 60) to powered wheels in order to move the vehicle 1. An important device in the power train is the gearbox 20, which has a number of forward gears for moving the vehicle 1 forwards, and usually also one or more reverse gears. The number of forward gears varies but modern kinds of trucks are usually provided with twelve forward gears.
The gearbox 20 may be of manual or automatic type (automatic gearbox), but also of the automatic manual gearbox type (automatic manual transmission, AMT). Automatic gearboxes and automatic manual gearboxes are automated gearbox systems usually controlled by a control unit 110 (sometimes also called electronic control unit, ECU) which is adapted to controlling the gearbox 20, e.g. during gear changing, as when choosing gears at a certain vehicle speed with a certain running resistance. The ECU may measure engine speed and the state of the gearbox 20 and control the gearbox by means of solenoid valves connected to compressed air devices. Information about the engine 10, e.g. its speed and torque, is also sent from the engine 10 to the ECU, e.g. via a CAN (controller area network) bus.
US Patent Publication 20080125946 refers to a method for determination of gear transmission ratio changes for an electronically controlled gearbox.
U.S. Pat. No. 5,479,345 further refers to a method and a device for choosing gearshift points. The factors of road gradient, the vehicle's speed and gross weight relative to the power, in hp, needed to maintain current running speed, and the engine speed and available power expected upon completion of a gear change are taken into account in determining whether a gear change is or is not allowed.
In conventional gear change systems, the control unit 110 uses tabulated engine speed limits, also called shift points, which represent the engine speed at which a downshift or upshift should be effected in the gearbox 20, i.e. the vehicle 1 changes gear when the speed of its engine 10 passes a speed represented by a shift point. The shift points may therefore be construed as providing information not only about when a downshift or upshift should take place but also about the number of gear steps to be effected at each downshift or upshift. It is usual for each shift point to comprise one to three gear steps, but more steps are possible.
FIG. 2 depicts schematically an example of various tabulated shift points represented by lines SP1-SP6 in a graph where the x axis represents engine torque and the y axis the speed of the engine 10 in revolutions per minute (rpm). So long as the engine speed is between shift lines SP1 and SP4 no gear change takes place, but if it rises above an upshift line, SP1-SP3, an upshift is initiated, and similarly a downshift is initiated if the engine speed drops below a downshift line, SP4-SP6. Table 1 below shows a number of upward or downward gear steps for each of the lines SP1-SP6. For example, an upshift by one step takes place if the engine speed rises above line SP1 and a downshift by two steps if the engine speed drops below line SP5.
TABLE 1Downshift and upshift lines SP1-SP6SP1Engine speed for upshift by 1 stepSP2Engine speed for upshift by 2 stepsSP3Engine speed for upshift by 3 stepsSP4Engine speed for downshift by 1 stepSP5Engine speed for downshift by 2 stepsSP6Engine speed for downshift by 3 steps
Shift point choices affect inter alia running characteristics, acceleration, comfort and fuel consumption for the vehicle 1, so shift points have to be accurately calibrated by vehicle manufacturers. This calibration involves various gearshift strategies being tested in the field in different driving situations, e.g. with different amounts of acceleration applied, different road gradients and different vehicle-combination weights.
A difficulty in choosing shift points is the desire for them to be appropriate to prevailing circumstances when the vehicle is in motion. For example, it is usual for a driver to change gear earlier on a downgrade than on an upgrade, since this makes it possible for the engine to cope with the climb, and it is also usual for the driver to change gear at higher engine speeds if the vehicle is heavily laden, since more power is then needed.